This invention relates to 1.) a composition of material which is capable of graphitizing cast iron in a highly effective and efficient manner, and 2.) the invention relates to combining and blending sulfur and oxygen compounds with other elements that are potent oxy-sulfide formers (such as rare earths, zirconium, calcium, zirconium, aluminum, barium, strontium, magnesium and titanium and which will heretofore be classified as `oxy-sulfide formers`, individually or in selected amounts), such blended compounds to be used to 1.) fabricate either a high density, inoculating `insert` or `tablet` or 2.) a granular or powder mixture of essentially the same composition which can be used as a direct addition to the molten metal.
The usual microstructure of gray iron is a matrix of ferrite and pearlite with graphite flakes dispersed throughout. Foundry metallurgical practices include `inoculating the metal` so that nucleation and growth of graphite flakes occurs in a pattern that enhances the desired properties. The `inoculating agent` can be added to either 1.) the pouring ladle, 2.) injecting or spraying the inoculant (in a finely divided or powdered form) into the metal pouring stream as the molten metals enters the mold, or as a insert placed in the mold. The amount, size and distribution of graphite are important to the physical properties of the gray iron. The use of inoculants to control microstructure as well as reduce the `chilling tendency or the formation of iron carbides (or cementite) is common practice in the ferrous foundry industry. The presence of "iron carbides" in the iron matrix is undesirable because this constituent is hard and brittle and can result in poor mechanical properties and machinability.
In ductile irons, the usual microstructure is a matrix of ferrite and pearlite with graphite nodules dispersed throughout the structure. Similar to gray cast iron, the nucleation and growth of the graphite nodules can be controlled by adding `post inoculants` to either the ladle, as an instream inoculant or as a insert placed at a strategic location in the mold. The size, shape and distribution of the graphite `nodules` is important to the physical properties of the ductile iron.
Inoculants can best be described as elements that can form stable compounds with either/or sulfur and oxygen, or both. These oxy-sulfide atomic clusters provide a substrate surface upon which dissolved graphite in the molten iron can "nucleate upon" or start to grow as graphite flakes or nodules, before sufficient undercooling occurs that favors the formation of "carbides".
Numerous metals and alloys have been proposed for use as inoculating agents in the production of both gray and ductile iron castings. Standard inoculating agents are 1.) calcium silicon, 2.) calcium bearing ferrosilicon alloys or other ferrosilicon based alloys that contain small percentages of oxy-sulfide forming elements and 3.) finely divided and powdered synthetic graphite.
In the manufacture of gray cast iron and ductile cast iron, it is almost essential to make an addition of an either a calcium bearing ferrosilicon or one of the more potent ferrosilicon inoculants containing relatively small percentages of oxy-sulfide forming elements prior to pouring the casting. In the case of the latter addition, these `oxy-sulfide` forming elements combine with dissolved oxygen and sulfur in the liquid iron. In almost all cases, the purpose of the ferrosilicon is to act as a `carrier` for the `oxy-sulfide forming elements` and ferrosilicon by itself provides little to no inoculating effect. Only certain amounts of these `inoculating capable elements` (or oxy-sulfide forming elements) can be technically and feasibly smelted and alloyed with ferrosilicon to produce commercially and economically available alloy products. This is largely due to the limited solubilities of the oxy-sulfide forming elements in liquid ferrosilicon alloys. It should be mentioned that ferrosilicon is used as the "carrier medium" because ferrosilicon is relatively inexpensive and dissolves quite easily when added to cast irons, thereby liberating through dissolution in the molten iron the small amounts of elements that react with dissolved oxygen and sulfur. Inoculation without using ferrosilicon inoculants but using oxy-sulfide forming elements was shown by the authors of "Minor Elements in Gray Iron", R. L. Naro and J. F. Wallace, American Foundrymen's Society, Transactions of the AFS, Volume 78, p. 229 (1970) ("Reference 1").
One such inoculant is known by the tradename of Superseed or Stronsil. This group of inoculants are strontium bearing ferrosilicon alloys containing small amounts of strontium (less than 1%) to promote Type A graphite flakes and minimize the formation of iron carbides. Another such ferrosilicon inoculant containing strontium, calcium and either zirconium or titanium is disclosed in U.S. Pat. No. 4,666,516. Another titanium ferrosilicon alloy, this one containing magnesium is disclosed in U.S. Pat. No. 4,568,388. Finally, inoculating alloys for gray iron are also known which include barium, e.g., U.S. Pat. No. 3,137,570 and 5,008,074.
Inoculants are commonly added to the metal pouring ladle prior to the actual casting process. A major problem in using any of the above inoculants as a ladle addition is that the inoculants' effectiveness diminishes rather rapidly after it is added. Thus, the first castings poured usually have improved microstructures and graphite structures versus those poured with metal from the same ladle only a minute or 2 later. This process of diminished effectiveness of inoculants with time at elevated metal pouring temperatures is know as `fade`. To circumvent `inoculant fade`, some of the same inoculating alloys are used in a powdery or granular form and injected into the metal stream just prior to entering the mold. These methods are usually more effective and normally much smaller addition levels need to be made. However, mechanical problems associated with the actual `injection` process as well as timing of the injected powder with the metal stream may be the source of inconsistent results and contamination of molding sands from `overspray` inoculants.
Inoculating in the mold is a third alternative, although it is not widely used. Either small lumps of calcium bearing ferrosilicon can be used or alternately, cast inserts made with ferrosilicon may be used. Since inoculation proceeds at the very last moment and virtually no time is available for fade, even smaller amounts of inoculant may be used as reported by the authors in "Chill Elimination in Ductile Iron by Mold Inoculation", W. Dell, Deere & Co. American Foundrymen's Society Publication `Conference on Modern Inoculating Practices for Gray and Ductile Iron`, Feb. 6-7, 1979, p. 283 ("Reference 2"). Efforts to make tablets with "inoculant containing materials" employing "molten wax" binders and have not met with commercial success. Reference 1 clearly states that wax bonded inoculants did not produce the desired effects. Recently, compacted and sintered fines of magnesium ferrosilicon and other silicon containing alloys have been produced in the shape of a tablet have been introduced to the market.
Traditional inoculants do not contain intentional additions of sulfur nor oxygen and must rely on the potential reaction of the `oxy-sulfide` forming elements which are added to traditional inoculants. Traditionally, all ferrosilicon based inoculants are smelted and refined in submerged arc furnaces and it is technically unfeasible to smelt sulfur and oxygen with these alloys because of liquid solubility constraints. It is also difficult if not impossible to incorporate significant amounts of these property enhancing elements (sulfur and oxygen) in traditional smelted ferroalloys.
The effectiveness of all inoculating agents is a direct function of the amount of sulfur dissolved in the molten irons and to a lesser extent, the amount of dissolved oxygen. The ability of `oxy-sulfide` forming elements to form nuclei assisting substrates, ie, oxy-sulfide atomic clusters, which in turn provides a similar crystalline surface onto which dissolved graphite atoms can precipitate from the liquid iron and grow is a necessary prerequisite for inoculation. Incorporation of sulfur and oxygen containing elements in the inoculant, thus insures that sufficient sulfur and oxygen will be available for subsequent reaction with the `oxy-sulfide` elements added as inoculants. Addition of these sulfide and oxygen compounds rejuvenates and beneficiates the molten iron and improves its responsiveness to inoculation.